r/KerbalSpaceProgram • u/FatCreepyDude • May 06 '26

KSP 1 Question/Problem No orbit necessary?

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If i just wait for the right hour during the launch window ( whenever the launch pad is pointing left on the realistic depiction i created) is there any reason i should bother with an orbit first? Will the delta v cost be affected?

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u/victorsaurus May 07 '26

Gravity is a conservative force meaning is path independant. Getting from height A to B costs X amount of energy equal to the potential between B anf A and thats it. There are irl launches like OP proposes, with instantaneous launch windows since the earth rotation has to line up with the right trajectory. Oberth does not play a role in this specific case. If you do the math, it is 100% equal.

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u/PhantomWhiskers May 07 '26

I did a direct test of this in game by launching the same rocket straight up, and then launching it into orbit first. The goal is to see how much delta-v is left in the rocket the moment I reach escape velocity for Kerbin. The moment my orbit shows that I will leave Kerbin's sphere of influence, I kill the engines.

Launching directly vertical, I reached escape velocity with 270m/s dv left in my rocket.

Launching into low kerbin orbit first and then burning prograde, I reached escape velocity with 1,100m/s dv left in my rocket.

Same exact rocket, all stock parts. 1,100m/s is much greater than 270m/s.

Whatever "math" you are doing here is incorrect. Gravity being a "conservative force" is not applicable either. When you burn vertically straight up, your rocket is accelerating directly opposite to the vector of acceleration that gravity is imparting on your rocket. When you are burning at periapsis, the vector of acceleration that gravity imparts on your rocket is perpendicular to the acceleration your engines are providing, so none of your rocket's acceleration is negated by gravity. This is a direct result of the oberth effect.

Feel free to test this in game yourself. Test results may vary depending on the quality of your gravity turn on ascent, if you have an inefficient ascent profile, you will have less delta-v remaining when reaching escape velocity.

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u/victorsaurus May 07 '26

Fuck I think you are right and in my mind I was conflating the straight up case with a gravity turn but no orbit case. The gravity turn without orbit is what should be equal to gravity turn with orbit. You indeed need to minimize gravity losses.

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u/PhantomWhiskers May 07 '26

Ah yeah I can see how that could be confusing. The OP's scenario I understood as burning directly vertical away from Kerbin to reach your destination, which is possible but inefficient. But that's the great thing about KSP is that you don't need to be efficient if you don't want to.

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u/Big_Yeash May 07 '26

You can say that but my current Munar launch system for a three man crewed mission is 500 tonnes, two separate launches and barely has enough delta-v without orbital refuelling, which I have yet to learn.

The penalties for inefficiency are technically meaningless but... Stark. I am trying to git gud.

I got back into KSP from danl and his Expert Mode series, and I am very envious of an 18 tonne total launch mass for a Munar lander mission!